The National Synchrotron Light Source at Brookhaven National Laboratory will be used to explore and develop the methodology for solving the phase problem for macromolecules using the multiple wave-length anomalous scattering method. Tunable wave-lengths in the range of 2.0 to 3.0 A will be used with Ca, Ba, or Cs derivatives of oligosaccharides and nucleotides as model compounds. The known structure of ferredoxin will be used to test the accuracy of the phase determination using tunable wave-lengths across the K-edge of Fe (1.7 to 1.9 A). When a successful methodology has been developed and tested, it will be used for problems which are unaccessible with standard or rotating target X-ray tubes. Such problems are the structure determinations of complexes of oligosaccharides or nucleotides which are only available as very small crystals, and the 28 Fe, 2 Mo atom structure of nitrogenase.